How could we determine how much time have passed without an artificial reference frame? Suppose one awakes from cryosleep in another region of the galaxy.
How s/he could tell how many years have passed since the beginning of the journey?
Observing cosmic expansion? Some kind of radioactive decay (on site)?
Restriction: One doesn't have a artificial reference frame from the journey's beginning, like ship's clock etc.
Think of it like a thought experiment: how to tell when we are in time based on natural phenomena. Sort of "Oh gosh, I slept too much. What time is it? I have no watch but let me look at the sun's position."
 A: There really is no one answer to this.
There is no such thing as absolute time. There is only time difference between the start and end. So you have to keep track of how far you have come in time in some way.
It is sort of like seeing how far you have travelled on a featureless plain. You have no way of knowing where you are. But you can put an X where you started, and measure how far from the X you are.
You can keep track of time by wearing a watch that you keep with you. Or by setting up a clock and reading the time from where ever you are.
But time doesn't travel at the same rate for everyone. You can different answers, depending on the route you take.
In special relativity, you always see time passing at the expected rate. But you see it passing slower for a moving person. This leads to the Twin Paradox. Two twins do the cryosleep thing. One stays on Earth. The other travels to Alpha Centauri and back at high speed. Both go to sleep when one twin leaves and wake up when he returns. Both twins check their watches. The moving twin sees less time passed.
Both would read the same time when checking a clock on Earth. That is, both would agrees on how much time has passed for the clock.
General relativity also has time distortions. A person in deep space far from massive objects sees time passing at the expected rate. He sees time passing more slowly for a person in a gravitational well, such as the surface of the Earth.
So two twins in deep space go to sleep. One travels to Earth and wakes up later. The other sleeps for a while and then travels to Earth, arriving just as the other twin wakes up. Both twins check their watches. The twin that has slept on slow moving Earth sees less time has passed. See Why can't I do this to get infinite energy? for an explanation.
